Why were all these things written?
These things were written because Paul is a prisoner of the Lord for us Gentiles, to reveal the mystery by revelation that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ by the Gospel, once kept secret but now unveiled to His holy apostles and prophets.
Les explains that when people begin to see what the Scripture says, it gets to them. In this context, Paul writes that all these things have been written because he is a prisoner of the Lord for us Gentiles. Then in verse 3 he calls it a revelation, a revealing: by revelation he made known the mystery, or secret, as he wrote before in a few words, so that when you read you may understand his knowledge as a result of revelations in the mystery of Christ. This mystery in other ages, generations, administrations, or dispensations was not made known. Les points out how plain this is in the light of Deuteronomy 29, Luke 18, and John 13, because God had been keeping it secret. Now it is revealed to His holy apostles and prophets that the Gentiles should be fellow heirs and partakers of His promise in Christ by the Gospel.